NES UART receive timeout

testloop:
      jsr get_char
      ldx #0
   @loop:
      lda buf,x
      jsr put_char
      inx
      cpx data_length
      bne @loop
      jmp testloop


;==================
get_char:
      sei
      lda #$40
      sta $4017
      lda #$80
      sta $4010
      lda #$3f
      sta $4013
      lda #0
      sta $4015
      lda #$1F
      sta $4015

      ldx #0
   @loop:
      lda #SERIAL_MASK
      cli   ; enable DMC long timeout
   @wait:
      bit $4017
      beq @wait

      ; 14 cycles
      sei
      lda #0
      sta $4010   ; disable DMC timeout, then enable frame IRQ short timeout with LSR $4017
      nop
      nop
      nop


.repeat 3
      lsr $4017
      ror a
      pha
      pla

      lsr $4017
      ror a
      nop
      nop
      nop
      nop
.endrepeat
      lsr $4017
      ror a
      pha
      pla

      lsr $4017
      ror a

      eor #$FF

      cmp #'Z'
      beq @exit
      sta buf,x
;      lda #0
;      sta $4010
;      sta $4017
      inx
      bne @loop

   @exit:
      stx data_length
      sei

      rts

;==================

IRQ:
      beep
      lda #'I'
      jsr put_char
      lda #'R'
      jsr put_char
      lda #'Q'
      jsr put_char
      lda $4015
      pla
      pla
      pla
      jmp testloop
      rti

;==================